Changing page style

I use Opera 12 for browsing. I've made a user style that I like and use it as default. Some times I have to change it for a page, so I set a keyboard shortcut for fast switch between author/user style. Τhat version of Opera is really old now, I have to use Firefox for some sites. To achieve the same goal with this browser, I downloaded the Stylish plugin and also used my style. Since there is no author/user shortcut in Firefox, I got keyconfig add-on and assigned the same shortcut to toggle Stylish on/off. With Vivaldi now, I couldn't find I shortcut to toggle a user style, so I went the Firefox way. Got Stylish, set it up, got keyconfig and ... problem! I can not find a way to assign a shortcut to stylish. Vivaldi doesn't provides extension shortcuts to the plugin like Firefox does. So that's my question: How can I assign a shortcut to a custom css style? TIA